1. Field of the Invention
The present invention relates to a foreign particle inspection apparatus, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
In a process of manufacturing an IC and an LSI, an exposure apparatus transfers a circuit pattern formed on a reticle onto a wafer coated with a resist.
During this transfer, if a pattern defect or a foreign particle or foreign substance such as dust is present on the reticle, it is transferred onto the wafer together with the targeted pattern, resulting in a decrease in the manufacturing yield. Especially when a circuit pattern is repeatedly transferred to a large number of shot regions on the wafer by exposure using the step & repeat scheme, if one harmful foreign particle is present on the reticle, it is also transferred onto the entire wafer surface by exposure. This results in a large decrease in the yield.
For this reason, it is essential to detect the presence of foreign particles on the reticle. To meet this need, a foreign particle inspection apparatus which exploits a property that foreign particles isotropically scatter light is generally used.
For example, a collimated light beam is obliquely projected onto the surface of an object to be inspected from above, and light scattered by a foreign particle is guided onto a one-dimensional image sensor (sensor array) by a gradient-index microlens array, thereby inspecting the surface of the object to be inspected (see Japanese Patent Laid-Open Nos. 7-043312 and 7-005115).
FIGS. 10A and 10B are views showing the basic arrangement of an optical system in a foreign particle inspection apparatus disclosed in Japanese Patent Laid-Open Nos. 7-043312 and 7-005115. For the sake of descriptive convenience, only an optical system for inspecting the blank surface of a reticle for foreign particles will be described herein. In practice, this apparatus also includes an optical system for inspecting, for foreign particles, a pellicle film which protects the circuit pattern surface of the reticle from foreign particles. Referring to FIGS. 10A and 10B, reference numeral 2 denotes a pellicle frame having a pellicle film attached on it.
A laser beam which is emitted by a semiconductor laser 41 and has a certain angle of divergence is collimated into a collimated light beam by a collimator lens 42. A λ/2 plate 43 guides the laser beam so that the polarization axis of the projected light is parallel to a plane including the optical axis of the projected light and that of the light received by a photo-receiving unit 7. The laser beam strikes the surface of an object to be inspected at an incident angle θ close to 90°. With this operation, the laser beam forms a linear light projection region 5 on a blank surface la as the surface of the object to be inspected.
If a foreign particle 3 is present in the light projection region 5, scattered light is generated by the foreign particle 3. This scattered light is converged on a line sensor 72 by an imaging lens 71 (lens array) which is formed by arraying lenses along the longitudinal direction of the light projection region 5 and serves to receive scattered light. The imaging lens 71 is configured to form an image of the light projection region 5 on the line sensor 72. The entire blank surface 1a is inspected for foreign particles by scanning an entire optical system 10 in a direction which is perpendicular to the longitudinal direction of the light projection region 5 and parallel to the blank surface 1a , i.e., by linearly scanning it in the X direction, as shown in FIG. 10B.
To increase the intensity of scattered light in proportion to increases in the particle size, the conventional foreign particle inspection apparatus sets the polarization axis of the laser beam in a direction nearly parallel to a plane including the optical axes of the projected light and the received light.
However, the minimum allowable particle size on the reticle or the pellicle in the recent exposure apparatuses is as small as about 10 μm. For this reason, it is becoming difficult to discriminate between a signal from light scattered by a 10-μm particle on an object to be inspected and that of light diffracted by the circuit pattern of the reticle when the foreign particle inspection apparatus inspects the object to be inspected. Note that the foreign particle inspection apparatus is set such that the intensity of scattered light increases in proportion to increases in the particle size, and that the discrimination performance between a signal from light scattered by a particle on the object to be inspected and that of light diffracted by the pattern improves. To attain this state, the λ/2 plate 43 included in a light projecting unit 4 (a unit which forms inspection light) optimally sets the azimuth of the polarization axis of the laser beam and that of the optical axis of the photo-receiving unit 7.
A phenomenon in which light diffracted by the pattern on the reticle is falsely detected as scattered light will be described next. FIG. 12 is a view when an optical path along which false detection often occurs is viewed from above the reticle and from the lateral side of a reticle side surface 1c (from the X direction). The light projecting unit 4 irradiates the linear light projection region 5 on the blank surface with a laser beam. Because the incident angle on the blank surface is relatively large, most (90% or more) of the light is reflected, but a certain component of the light enters the reticle. At this time, as the light is refracted at a position P on the blank surface and irradiates a linear circuit pattern 102 extending in the Y direction, pattern-diffracted light 103L and pattern-diffracted light 103R are generated by the circuit pattern 102.
When the circuit pattern 102 is obliquely irradiated with the light, diffracted light scatters in a direction perpendicular to the circuit pattern 102 with reference to light regularly reflected by the circuit pattern 102. Light which enters the reticle at the position P upon striking the reticle at an incident angle close to 90° and being refracted at that position serves as pattern irradiation light. If the light is further diffracted by the pattern, the diffracted light is totally reflected by the blank surface even upon reaching it. Likewise, when the light totally reflected by the blank surface reaches the pattern surface, it is totally reflected in the incident region on the pattern surface again if the circuit pattern is absent in this region. In addition, depending on the density of the circuit pattern 102, the light is often totally reflected by a back side surface 1b of the reticle and the reticle side surface 1c. In this manner, unless the circuit pattern 102 is illuminated again to cause a diffraction phenomenon, the pattern-diffracted light 103L is often totally reflected by all of the pattern surface (irrespective of a light-shielding film portion, a glass portion, or a semitransparent film portion), the blank surface, and the reticle side surfaces.
Also referring to FIG. 12, if the pattern-diffracted light 103L repeatedly undergoes total reflection, it often returns to a position below (in the Z direction) the light projection region 5. If a linear pattern 104 extending in the X direction is present at this position, diffracted light 105 is often generated by it again and received by the photo-receiving unit 7. This phenomenon will be explained with reference to FIG. 13.
FIG. 13 is a view when the optical system shown in FIG. 10A is viewed from the side of the light projecting unit 4. Of the optical path along which the pattern-diffracted light 103L which repeatedly undergoes total reflection travels, that from the reticle side surface 1c to the pattern 104 is indicated by a dotted line. The pattern 104 is a linear pattern in the X direction, so the diffracted light 105 corresponds to that obtained by changing the tilt about the X-axis, which is perpendicular to the pattern 104, with reference to regularly reflected light. For this reason, the diffracted light 105 appears as if it overlapped regularly reflected light in FIG. 13, but FIG. 12 reveals that its incident angle on the blank surface is often smaller than a critical angle and therefore the light emerges into the air. Furthermore, depending on the density of the linear pattern 104, the diffracted light 105 is often diffracted at an angle nearly matching the direction of the optical axis of the imaging lens 71 of the photo-receiving unit, and detected as light scattered by a foreign particle by the line sensor 72.
The pattern-diffracted light 103R repeatedly undergoes total reflection as well, but it is gradually attenuated upon entering a circuit pattern region 101 on the side surface 1b because diffracted light is generated. The generated diffracted light never enters the photo-receiving unit 7, so it is never detected falsely.